Computer tomography (CT, also called computed tomography) has evolved into a commonly used means, when it comes to generating a three-dimensional image of the internals of an object. The three-dimensional image is created based on a large number of two-dimensional X-ray images taken around a single axis of rotation. While CT is most commonly used for medical diagnosis of the human body, it has also been found applicable for non-destructive materials testing. Detailed information regarding the basics and the application of CT, can be found in the book “Computed Tomography” by Willi A. Kalender, ISBN 3-89578-216-5.
One of the key innovative aspects in future CT and X-ray imaging is the energy-resolved counting of the photons which are let through or transmitted by the object being analyzed when being exposed to X-ray radiation. Depending on the number and energy the transmitted photons have, it can be concluded, after a slice image reconstruction step, through which types of material the X-ray beams have traveled. In particular, this allows identifying different parts, tissues and materials within a human body.
When the detection or counting of photons is referenced, it is understood, that when a photon impinges on the conversion material of a sensor, it creates a charge pulse. This charge pulse (sometimes also referred to as current pulse) is detected and the presence of a photon is concluded. The charge pulse results from a larger number of electron-hole pairs, which are generated, when an X-ray photon interacts with the sensor conversion material. Since the charge pulse that is processed corresponds to the X-ray photon, the processing of the charge pulse is also referenced as “processing an X-ray photon” or as “processing a photon”.
Great interest is shown toward CT with energy-dispersive features, because it is perceived that this will enable new applications in X-ray tomographic imaging, in particular with regards to K-edge imaging of contrast agents (Gd, Au, Bi, etc.). Furthermore, energy-dispersive CT, also called Spectral-CT, allows for quantitative imaging of μ-values, while being more dose-effective than conventional X-ray CT. Simulations on a high level of abstraction show that a Spectral-CT scanner based on energy-dispersive single-quantum counting yields the best performance as of today. However, the realization of such a detector is not straight-forward and it is difficult to deal with energy-dispersive X-ray detection for a large dynamic range of X-ray intensities.
The counting of a single quantum can be handled quite well in those parts of the detector where the X-ray beam is strongly attenuated by the object to be scanned. This is due to the fact that the flux density is significantly lower than the flux density emitted from the X-ray source. However, detecting almost unattenuated radiation in counting mode presents a challenge in X-ray CT, since the scanner has to deal with more than 109 quanta/mm2/s. Given such a high rate of incoming photons, it becomes very difficult to differentiate the individual photons and to accurately count them.
For instance, the detector will be busy while charges are collected in the sensor and while the analog signals are processed in the electronics. This leads to a significant decrease in the event-detection efficiency. In order to address this, correction factors can be applied that are in the order of 200%. However, since the ideal correction factor required for estimating the true event rate at the detector depends on the magnitude of the incoming X-ray photon flux, accurate results cannot be expected.
Another aspect that has to be considered is known as “event pile-up”, where hits of subsequent X-ray photons in the detector lead to a pile-up in the detector. This causes a distortion of the measured X-ray spectrum. Using simulations it has been shown that this effect is quite severe for high energy levels, in particular for the energy channels above 100 keV.